The standards of the Advanced Television System Committee (ATSC) suggest to use a signal obtained by modulating 12 independent data streams, which are trellis encoded and time-multiplexed, into 10.76 MHz-rate 8-level Vestigial Side Band (VSB) to transmit High Definition Television (HDTV) broadcasting through a terrestrial broadcasting channel. The frequency band of the signal is transformed into a frequency band of 6 MHz which corresponds to a standard Very High Frequency (VHF) or Ultrahigh Frequency (UHF) terrestrial television channel. Signals of the corresponding channel are broadcasted at a data rate of 19.39 Mbps. Detailed technology on the ATSC DTV standards and A/53 are available at http://www.atsc.org/.
FIG. 1 is a block diagram showing a conventional DTV transmitter. As shown, data inputted into a transmitter 100 are serial data streams formed of 188-byte Moving Picture Experts Group (MPEG) compatible data packets, each of which includes a synchronous byte and 187-byte payload data. The inputted data are randomized in a data randomizer 101 and each packet is encoded to include 20-byte parity information for forward error correction (FEC), FEC-Reed Solomon (RS) coding, 1/6 data field interleaving, and 2/3 trellis coding. That is, according to the ATSC standards, the data randomizer 101 performs XOR on the payload data bytes inputted to a pseudo random binary sequence (PRBS) having a maximum length of 16 bits, which is initialized at a starting field of a data field.
In the RS encoder 103 receiving the outputted randomized data, data having a total of 207 bytes are generated for each data segment by adding 20 RS parity bytes for FEC to the 187 bytes.
The randomization and FEC are not performed on synchronous bytes corresponding to a segment synchronous signal among the inputted packet data.
Subsequently, data packets included in consecutive segments of each field are interleaved in a data interleaver 105, and the interleaved data packets are interleaved again and encoded in a trellis encoder 107. The trellis encoder 107 generates a stream of a data symbol expressed in three bits by using two bits input. One bit of the inputted two bits is pre-coded and the other bit is 4-state trellis encoded into two bits. The three bits finally outputted are mapped to an 8-level symbol. The trellis encoder 107 includes 12 parallel trellis encoders and precoders to generate 12 interleaved/coded data sequences.
The 8-level symbol are combined in a multiplexer (MUX) 109 with segment and field synchronization bit sequences 117 from a synchronization unit (not shown) to form a transmission data frame. Subsequently, a pilot signal is added in a pilot adder 111. Symbol streams go through VSB suppressed-carrier modulation in a VSB modulator 113. An 8-VSB symbol stream of a baseband is finally converted into a radio frequency (RF) signal in an RF converter 115 and then transmitted.
FIG. 2 is a block diagram describing a conventional DTV receiver 200. As illustrated, a channel for the RF signal transmitted from the transmitter 100 is selected in a tuner 201 of the receiver 200. Then, the RF signal goes through intermediate frequency (IF) filtering in an IF filter and detector 203 and a synchronous frequency is detected. A synchronous and timing recovery block 215 detects a synchronous signal and recovers a clock signal.
Subsequently, the signal is removed of a National Television Systems Committee (NTSC) interference signal through a comb filter in an NTSC removing filter 205, and equalized and phase-tracked in an equalizer and phase tracker 207.
An encoded data symbol removed of multi-path interference goes through trellis decoding in a trellis decoder 209. The decoded data symbol is deinterleaved in a data deinterleaver 211. Subsequently, the data symbol is RS decoded in an RS decoder 213 and derandomized in a data derandomizer 217. This way, the MPEG compatible data packet transmitted from the transmitter 100 can be restored.
FIG. 3 is a diagram illustrating a transmission data frame exchanged between the transmitter of FIG. 1 and the receiver of FIG. 2. As illustrated in the drawing, a transmission data frame includes two data fields and each data field is formed of 313 data segments.
The first data segment of each data field is a synchronous signal, i.e., a data field synchronous signal, which includes a training data sequence used in the receiver 200. The other 312 data segments include a 188-byte transport packet and 20-byte data for FEC, individually. Each data segment is formed of data included in a couple of transmission packets due to data interleaving. In other words, the data of each data segment correspond to several transmission packets.
Each data segment is formed of 832 symbols. The first four symbols are binary and they provide data segment synchronization. A data segment synchronous signal corresponds to a synchronous byte, which is the first byte among the 188 bytes of the MPEG compatible data packet. The other 828 symbols correspond to 187 bytes of the MPEG compatible data packet and 20 bytes for FEC. The 828 symbols are transmitted in the form of an 8-level signal, and each symbol is expressed in three bits. Therefore, 2,484 bits (=828 symbols×3 bits/symbol) are transmitted per data segment.
However, transmission signals of a conventional 8-VSB transceiver are distorted in indoor and mobile channel environments due to variable channel and multipath phenomena, and this degrades reception performance of the receiver.
In other words, transmitted data are affected by various channel distortion factors. The channel distortion factors include a multipath phenomenon, frequency offset, phase jitter and the like. To compensate for the signal distortion caused by the channel distortion factors, a training data sequence is transmitted every 24.2 ms, but a change in multipath characteristics and Doppler interference exist even in the time interval of 24.2 ms that the training data sequences are transmitted. Since an equalizer of the receiver does not have a convergence speed fast enough to compensate for the distortion of receiving signals, which occurs by the change in multipath characteristics and the Doppler interference, the receiver cannot perform equalization precisely.
For this reason, the broadcasting program reception performance of 8-VSB DTV broadcast is lower than that of an analog broadcast and reception is impossible in a mobile receiver. Even if reception is possible, there is a problem that a signal-to-noise ratio (SNR) satisfying Threshold of Visibility (TOV) increases.
To solve the problems, International publication Nos. WO 02/080559 and WO 02/100026, and U.S. Patent publication No. US2002/019470 disclose technology for transmitting robust data by any one among 4-level symbols, e.g., {−7,−5,5,7} or {−7,−3,3,7}. Since the symbols to which robust data are mapped are limited in the conventional technology, there is a problem that the average power of the symbols corresponding to the robust data is increased compared to conventional 8-VSB method. In other words, when robust data are transmitted by any one among four level symbols {−7,−5,5,7}, symbol average power is 37 energy/symbol, or if robust data are transmitted by any one among four level symbols {−7,−3,3,7}, symbol average power is 29 energy/symbol, which signifies that the average power of the symbol corresponding to the robust data is increased compared to the conventional 8-VSB method. The increase in the symbol average power leads to increase in the entire average power. When signals are transmitted with a limited transmission power, which is true in most cases, the transmission power of normal data are relatively reduced compared to the conventional 8-VSB method and, thus, there is a problem that the normal data have poorer reception performance than the conventional 8-VSB method in the same channel environment.
Since the problem becomes more serious when the ratio of robust data mixed with normal data is increased, the SNR satisfying the TOV is increased. Accordingly, the reception performance is degraded, even though the channel environment is fine and it is likely to happen that backward compatibility cannot be provided for an 8-VSB receiver.